The structure sensitivity of copper catalysts supported on monoclinic and tetragonal ZrO2 was studied on the water gas shift (WGS) reaction. The results show that the activity of copper supported on tetragonal ZrO2 is much higher than when supported on monoclinic ZrO2. The obtained results favour an associative mechanism, which considers the formation of formate species on the surface of ZrO2. The differences on the observed WGS activity are attributed to the stability of adsorbed formates on the different ZrO2 polymorphs. It was found that the active Cu species for the WGS reaction is the bulk CuO.